Conventionally, in observation optical systems used in, for example, magnifiers, binoculars, and terrestrial telescopes, there has commonly been adopted a so-called Keplerial type (real-image type) arrangement, in which an inverted image formed by an objective system is converted into an erect image by an erecting system such as a prism, and the erect image is observed through an eyepiece system. On the other hand, in zoom optical systems having a zoom ratio of about 2×, which are easy to make compact, there has commonly been adopted a zoom type arrangement, in which zooming is performed by a pair of lens elements between which the inverted image is located. Patent Literature 1, for example, proposes a variable-magnification optical system in which the most pupil-side lens group in the eyepiece system is composed of cemented lens elements for the purpose of correcting longitudinal chromatic aberration.